Walking through the Dupont Circle Metro station on a Saturday afternoon this spring, I was stopped in my tracks by the calm tones of an announcement that, for the convenience of Metro's passengers, the system would open at 9 a.m. the next day instead of the usual 10 a.m. Actually, the announcement went on to admit, though not in so many words, this change wasn't for the benefit of all Metro's passengers. Rather, the schedule change was "in honor of the Centennial Jamboree of the Girl Scouts of America."

It turned out that the Girl Scouts organization had called its jamboree in downtown Washington and then realized that little girls and their troop leaders would be converging there from all over Maryland and Virginia before the service opened. Somebody in the organization had been savvy enough to give Metro a call. Metro officials say the extra hour of service probably cost only five or six thousand dollars -- a small price to pay for regional good will.

It's hard to complain about Metro in Washington without sounding like a crank. It's particularly hard when you're from New York and can expect even the mildest observation about service to be thrown back in your face. I'm willing to yield the major points on quality. But this is a mass transportation system, one of the few types of product where quantity is as important as quality. If the Girl Scouts of America can get public transportation early on Sunday morning, why can't we all?

It's not that Metro doesn't mean well. It's been slowly increasing its hours for years, up to the current 6 a.m.-to-midnight weekdays, 8 a.m.-to-midnight Saturdays and 10 a.m.-to-midnight Sundays -- not bad considering the original weekdays-only service that closed at 8. But the shortfalls in the schedule are still glaring. They glare on weekend nights, when the midnight closing hour produces a "Cinderella Syndrome" in which all your guests suddenly get up and rush out the door. They also glare on Sunday mornings, if you happen to work a weekend shift or you want to go to church.

Metro's Office of Planning wants to please -- maybe too much. It makes exceptions for all kinds of things, staying open late for big events at RFK Stadium and Wolf Trap, for holidays, sometimes for marches or rallies. "If we get a large number of people to an event," says Metro public affairs director Beverly Silverberg, "we feel an obligation to get them all home." How do they make sure they hear about all the events that might need service? Well, they don't. "We try to stay wired, and find them before they find us," says Harry Barley, manager of system and service planning. "We usually just know."

There are, however, gaps. Not everyone knows, as somebody connected to the Girl Scouts did, that a phone call to Metro with a seemingly audacious request ("Could you open your whole city transportation system just for us?") can bring results. One of the people who doesn't know it is Joan Corboy, the regional coordinator for the national organization Mothers Against Drunk Driving. MADD wants to cut down on teen-age drunk-driving fatalities, and extending Metro hours on weekends so that fewer people drove home from bars might very well do that, but Corboy says she's new in town and never thought of it. Others who probably haven't tried are restaurant workers, sanitation workers, hotel workers or anybody else with odd hours.

"There's nothing magic about midnight," says Metro board member and longtime booster Carleton Sickles. The original financial blueprints for the system, he says, assumed an ultimate schedule of 5 a.m. to 1 a.m. seven days a week. Each step toward that goal is a new compromise between the constraint of cost -- an extension from 6 a.m. to 5 a.m. alone would cost about $3.5 million a year, according to planners -- and the push of rider demand.

Until June 1986, for example, Metro closed at 6 p.m. every Sunday, a ridiculous cutoff time which especially stranded weekend travelers. For several years, the system dealt with the problem by constantly making special exceptions. Then, Barley says, they realized that not only were they losing money whenever they made a wrong guess, they were also losing the large number of riders who would have ridden Sunday nights if they'd only known Metro was going to be open. Now the riders know, and more and more of them ride.

How much more would a 24-hour system cost? That's hard to answer. Metro officials say you can't extrapolate from the cost of an occasional one-hour extension, and no one has come up with a price tag -- the constraints of the union contract and maintenance make it a difficult guessing game.

"The service to some extent reflects the town," Silverberg says, and Washington, despite great changes in the past 10 years, "is still a town where everybody, unless there's a party, is home in time for the 11 o'clock news." But that's only one Washington, the one the Metro planning office knows well enough to serve just by being "wired in." If Metro stopped giving favors to people it knows could use late service, early service or Sunday service, it might find that a lot of the people it doesn't know could use it too. Amy E. Schwartz is a member of the editorial page staff.